Ceramic materials for cutting tool applications include alumina, alumina-zirconia, alumina-TiC-TiN, silicon nitride, sialon, and SiC-whisker reinforced alumina. The cutting tool environment puts simultaneous high demands on the strength, toughness and thermal shock resistance in addition to the obvious demands for high wear resistance.
The mechanical properties of ceramic materials are to a high extent influenced by internal and external defects, such as inclusions of foreign matter, pores, large grains and cracks. In order to improve reliability and performance of cutting tools made of ceramic materials, it is necessary to identify detrimental defects in the products and to set up the processing route in order to minimize undesirable features. Since ceramic materials have a completely elastic behavior up to temperatures of about 1000° C., the stress concentrations created by the defects cannot be eliminated by relaxation due to plastic deformation.
Ceramic materials for metal cutting tools are produced by milling of the constituents in a liquid and subsequent drying of the slurry. Spray drying is the preferred drying method for materials that do not require hot pressing. Spray drying produces granules with a size of 50–200 microns. The large granules give very good powder flow properties, which is essential for mass production of blanks with uniaxial cold pressing.
It is now well-established that defects in a sintered body can be related to the pore size and distribution in the green compact. This is especially important for materials that are sintered without pressure or with low pressure (gas pressure sintering), since large pores will not be eliminated. The granule characteristics, and especially their deformation behavior, will be the primary parameters that determine the defect structure in the green state. Considerable increases in strength of the sintered material have been achieved by reducing the granule compression strength, since dense and hard granules will retain their shape even after compression.
Besides pore size and pore distribution, the grain size is also essential to the mechanical properties of ceramic materials. A fine and uniform grain size provides for a high strength and a small variation of the strength. The grain size of ceramic materials is closely related to the sintering conditions.
Alumina and alumina-zirconia materials are preferably produced by ressureless sintering in an appropriate atmosphere. In many cases, this is the preferred sintering technique for ceramic materials, since it is a relatively low-cost process and enables complex shaped parts to be produced.
Silicon nitride and sialon materials are normally produced by gas pressure sintering, whereby a gas pressure of from about 0.1 to about 1 MPa is applied, once closed porosity is reached in the material. This enables higher densities to be reached at lower sintering temperatures, especially when using low amounts of sintering additives to form a liquid phase.
Hot isostatic pressing (HIP) is another sintering technique that is used for materials that cannot be consolidated without external pressure. Pressures of from about 1 to about 10 MPa are normally used, but the method demands an encapsulation, for example in glass, to transmit the gas pressure. HIP can also be used to remove remaining porosity after conventional sintering or hot pressing to closed porosity, the advantages being that HIP, due to the high pressure, is performed at a lower temperature than the sintering temperature, which is why a more fine grained material is obtained.
Hot pressing (HP) is the preferred method for materials difficult to be sintered, like silicon whisker reinforced alumina, and also for mixed ceramics, like alumina-TiC. The pressure of normally from about 25 to about 35 MPa is uniaxially transferred to the material with graphite punches. Rather large cylindrical discs are obtained, which are then diamond saw cut to the required dimensions of the blanks. The diamond saw cutting is a rather expensive part of the blank production process, amounting to from about 30 to about 40% of the production costs per blank.
U.S. Pat. No. 4,543,345 describes a method for the production of silicon carbide whisker reinforced alumina with from about 5 to about 60% by volume SiC-whiskers to a sintered density of greater than 99%. The process requires a pressure of from about 28 to about 70 MPa, a temperature of from about 1600 to about 1900° C., and a hold time at sintering temperature of 45 minutes to 2 hours. Pressures and sintering temperatures in the higher range are needed for higher whisker loadings. The combination of long sintering times and high sintering temperatures in this hot pressing sintering method leads to alumina grain growth in spite of the grain growth inhibiting effect of the silicon carbide whiskers. Large alumina grains will affect he performance in cutting tool applications, since the largest defect determines the strength of the material.
Another method, spark plasma sintering (SPS), applies electrical energy pulses directly to the gaps between the powder particles, which are placed between graphite punches. SPS utilizes the energy of the spark plasma generated by the spark discharges. The pressure is directly applied on the powder bed in an uniaxial direction.
Another method uses a particulate solid as the pressure-transmitting medium, which is why such method is referred to as “pseudo-isostatic.” Such method can be used to consolidate preforms of more complicated shape.
U.S. Pat. No. 5,348,694 describes a sintering method, wherein the preformed green blank is heated by electrical resistive heating of a granular pressure-transmitting medium, which is in contact with the preform inside a die chamber. The pressure-transmitting medium is electrically conductive, e.g., graphitic carbon granules. This electrical resistive heating method enables very high temperatures and rapid heating times, making it suitable for materials that require high sintering temperatures. The pressure that can be applied is limited by the strength of the material in the rams and die, which for high temperatures is normally graphite. The pressure is therefore usually not much higher than about 100 MPa.